Application Example: Compounding and Extrusion of Wood-Plastic Composites

The production of Wood-Plastic Composites typically uses a fine wood waste (cellulose based fiber fillers such as hardwood, softwood, plywood, peanut hulls, bamboo, straw, etc.) mixed with various plastics (PP, PE, PVC). The powder is extruded to a doughlike consistency and then extruded to the desired shape.

Additives such as colorants, coupling agents, stabilizers, blowing agents, reinforcing agents, foaming agents, and lubricants help tailor the plastic end product to the target area of application.

With up to 70 percent cellulose content, wood-plastic composites behave like wood using conventional woodworking tools. At the same time, they are extremely moisture-resistant. There is little or no water present, thus increasing resistance to rot.

Wood-plastic composites are already widespread in outdoor use for decking, cladding, park benches, etc. There is also a growing market for potential indoor uses such as door frames, trim and furniture.

Products include: Lumber, decking and railing, window profiles, wall studs, door frames, furniture, pallets, fencing, docks, siding, architectural profiles, louvers, automotive components

Characteristics

• Fastest growing sector of plastics industry
• Volume processors must produce faster, better, cheaper
• Weatherability and life cycle costs are major factors
• Formulation variations that increase wood content offer expansion into other uses
• Environmentally safe and efficient

Key Blending Considerations in Wood-Plastic Composites

Compared to many other processes, on-line compounding and extrusion exhibit short performance timescales, consistent with the mixing time of the extruder. For high formulation quality the feeding system must achieve and maintain its required blending accuracy within this brief time.

Key to a feeder's ability to attain high accuracy over short intervals is the resolution of its weighing device and response of its process controller.

As the illustration at right shows, lengthy performance timescales permit a given blending accuracy to be attained with a low performance weighing system.

However, to achieve the same accuracy in the short interval characteristic of compounding and extrusion operations, a much higher weighing performance is required.

Additional important blending system considerations in wood/plastic composite processing include control and weigh system insensitivity to environmental disturbances, and the ability to reliably handle/control difficult organic components.

1. Pre-dried wood along with resin/regrinds and additives
all enter at the throat of a twin screw pelletizing extruder.

2. Pre-dried wood enters at twin screw pelletizing extruder throat with resin/regrind and additives introduced along the barrel.

3. Continuous mixer discharges dry wood at twin screw extruder throat with resin/regrind and additives introduced along barrel to produce end product profile.

4. A high-speed compounding extruder supplies multiple single or twin screw profiling extruders. Alternatively, compounding extruder feeds a gear pump to produce the profile.

5. Wood enters twin screw extruder throat and is dried within the extruder.

6. Multi-extrusion application with wood and resin, regrind, and additives on main twin screw extruder with single screw extruders discharging to die head.

With the diversity of organic components used in wood/plastic composite processing, there is no single answer to reliably handling these potentially difficult materials.

Pre-purchase testing helps determine the optimal combination of bin design, agitation and other flow aid strategies for the specific material in use.

K-Tron's fully-equipped testing facilities are available to evaluate your materials and determine the optimal combination of equipment components to assure the highest level of accuracy and reliability. Computerized performance test reports document equipment performance.

Feeder design and agitation solutions are also key to handling difficult bulk materials. K-Tron offers over 30 years of experience in material handling technology.